Metal diffusion coating utilizing fluidized bed



May 3, 1966 c. H. JUNG E'rAl. 3,249,462

METAL DIFFUSION COATING UTILIZING FLUIDIZED BED Filed oct. 2:5, 1961 o l I /400 /faa /cao /700 /oo /sao COAT/fm 7Z-wPE/mz//7E INVEN TOR.

A TTFIYEYS United States Patent O y 3,249,462 METAL DIFFUSION COATING UTLIZENG FLUIDIZED BED Charles H. Jung, Seattle, Ross J. Wood, Edmonds, and lChristopher L. Lofgren, Seattle, Wash., assignors to vThe Boeing Company, Seattle, Wash., a corporation of Delaware Filed Oct. 23, 1961, Ser. No. 147,004

Claims. (Cl. 117-71) The present invention relates to the diffusion coating of metals and in particular to metal diffusion coating techniques utilizing a uidized bed. In a still more limited sense, the invention has particular application to the formation of a disilicide diffusion complex on silicide forming refractory metals by such means.

. Refractory metals such as molybdenum, tungsten, tantalum, columbium, zirconium, chromium, titanium,` vanadium, and hafnium are currently under consideration for use in fabricating, or for incorporation into super alloys and heat resistant alloys used in fabricating the leading edges, under-side panels, and similarly exposed skin sections of space vehicles and missiles which experience excessively high skin temperatures during reentry into the earths atmosphere. Despite any high emissivity characteristics the skin of such vehicles and missiles may have, at least portions of it are expected to experience surface temperatures ranging as high as 3000 F. or more. At `such temperatures all refractory metals are susceptible to oxidation, corrosion, and various other forms of oxidative contamination when exposed to the earths atmosphere. vMolybdenum, for example, experiences catastrophic oxidation in an oxidative atmosphere at temperatures abovev 1300 F. It is recognized, therefore, that these skin metals will require a protective surface coat capable of withstanding atmospheric conditions at these temperatures. y

An object of the present invention is to provide techniques for applying such a protective surface coat to a metallic substrateA and 'particularly to a refractory metal substrate.

g Another object of the present invention is to provide techniqueszfor applying a tightly adherent protective surface coat on a metallic substrate, which coat is adapted to resist high temperature oxidation and corrosion, and particularly oxidative conditions at temperatures in the magnitude of 3000 F. or more.

v A third object of the invention is to develop techniques for cladding a metallic base with a protective diffusion complex thereof, utilizing a fluidized bed.

A further object of the invention is to provide techniques for diffusion coating a silicide-forming refractory metal base, utilizing a iluidized bed.

A further object of the invention is to provide techniques by which the y-coating operation can or may be materially accelerated.

- A stilly further object of the invention is to provide vapor deposition coating techniques which are technically and economically feasible on a mass production scale. In this connection theinvention is especially intended to overcome the drawbacks of pack methods for providing vapor deposition coatings. These include 'contamination of the work piece, interaction of the work piece with catalytic materials present during thermal buildup and treatment,-distortion of the work Ipiece during `this time,

a tendencyon the part of the metallic base to embrittle,`

andl a tendency on the part of the base to form oxide inclusions, to develop porosity, and to lose its adhesion with the protective layer.

Still other objects include the development of techniques which avoid the problems of reproducibility and reliability encountered with the pack methods, and techniques which permit coating complex shapes such as corcompound of silicon.

3,24%,452 Patented May 3, 1966 -tion also is designed to provide techniques for applying a protective coat which has improved emissivity.

In reaching these and other objects the invention makes use of a fluidized bed and concepts involving a facility of this type. The iluidized bed, in its simplest form, consists of a container holding a bed of particulate material which is agitated by passing a gas upward through the material. When the llow rate of the gas causes the agitated particles to lose Contact with one another and to become fluid borne, the bed is said to be fluidiz'ed.

According to the invention, a refractory metal article is coated with a -silicide diffusion complex thereof, by

immersing the article in a bed of fluidized particulate material while permeating the bed interstices around the article with a vaporized thermally decomposable cornpound of silicon, and maintaining the bed in the iluidized condition about the article while adjusting its temperature until the compound decomposes and forms a coating of the complex onthe article. In most instances the vaporized compound is a thermally decomposable halide Moreover, under our preferred practice, the silicon is contained in the particulate material and the vaporized compound of the same is produced by introducing a halogen gas into the bed at a temperature which causes the gas to react with the silicon in the particulate material to form the halide thereof. -The halogen gas may be introduced with a iluidizing gas.

Several techniques have also been discovered for accelerating the coating rate and/or improving the oxidation resistance of the coating. One of these involves preplating the refractory metal article before the coating operation with a metal which increases the rate at which the silicon alloy of the article forms thereon. The preplated metal may be applied by electrolytic or vapor deposition. It is also contemplated that the preplated metal may be another refractory metal, such as molybdenum,

which forms a more resistant alloy than that formed from the refractory substrate alone. For example, columbium, which has highly desirable formability and weldability characteristics, but whose alloy with silicon is not as oxidation resistant as molybdenum disilicide, can be preplated with molybdenum to form the more resistant molybdenum alloy in the coating operation.

Another technique for accelerating the coating rate of silicon on molybdenum, while obtaining the more resistant disilicide complex of the molybdenum substrate, in Volves a so-called duplex process wherein a silicide coating is formed in one temperature range and then converted to a disilicide complex in a higher range. Preliminarily, it should be mentioned that the oxidation resistance of disilicide coated refractory metal articles is a function of coating thickness, which in turn is normally a direct function of coating time and temperature, inasmuch as the coating rate generally follows an ever-increasing slope as temperature increases. l However, in coating molybdenum articles, it has been observed that the article has a characteristically sinusoidal coating rate with increasing coating temperature, in that the silicon is deposited on the article at a peak coating rate in a relatively low temperature range and then, as the coating temperature is raised above this range, is deposited at a progressively lower coating rate untila second critical temperature is reached at which the coating rate is at a minimum.

Thereafter it reverses and becomes progressively greater toward another peak as the coating temperature is raised further. t would seem from this that an advantage could be gained from forming. the coat on a molybdenum article in the low temperature range where a coat having the desired resistance seemingly could be built up more rapidly. However, the coat formed in such an instance has not proven .to have a characteristic oxidation resistance and, in fact, has not been observed to be a true disilicide coat. The coat formed at higher temperatures, on the other hand, can be identified conclusively as a disilicide coat having a characteristic oxidation resistance which increases with increasing coat thickness. The ternperature at which the characteristic disilicide coat begins to form varies with any alloying thereof.

As observed in the accompanying graph, wherein the abscissa indicates coating temperature F.) and the ordinate indicates coating thickness (mils/side), the rate of silicon deposition'on a Mo 0.5% Ti article increases to a maximum in a relatively low temperature range of l475-1600 F. and then decreases to a minimum at or about 1800 F., .from which it again rises with increasing coating temperature. In the 1475-1600 F. range the coating is not a characteristic MoSi2 complex and may comprise or consist of a Mo3Si2 phase. X-ray diffraction tests show that MoSi2 formation begins at or about 1700 F. and becomes substantially complete at or about 1800 F. At this last temperature, however, the coating rate is -at a minimum. Only at some still higher temperature-at which alloy formation with the material of which the bed facility is made raises problems-does the coatinglrate again compare with that in the low temperature range.

According to the invention, therefore, the molybdenum article is immersed in a bed of iuidized particulate material while the bed interstices around the article are permeated with a vaporized halide compound of silicon, and the temperature of the bed `is adjusted, firstly, to the range of 1475 `1600 F., so as to decompose the compound and `form a coating of the silicide diffusion complex on the article, and thereafter, to a temperature of at least` 1800 F. to convert the coating to the disilicide diffusion complex of molybdenum. This so-called duplex process enables the coating to be applied at the most rapid rate without sacrificing protection against oxidation in the result.

Example I In an example of the inventive techniques a Mo 0.5 Ti leading edge was siliconized in an 8-inch uidized bed. Silicon metal (98% minimum pure grade, mesh size -20, +80) was added to the bed facility to an unuidized depth of 14 inches, provision being made for adding more as required to maintain a uidized bed depth of 16 to 18 inches at operating temperature. Argon from an argon generator was introduced to the diffusion plenum and` released upwardly into the silicon bed through a porous ceramic diffuser until the bed reached a fluidized condition. The bed was then heated to 1800 F. with a luminous wall furnace. The leading edge part was cleaned and then lowered into the fluidized bed. An iodine retort was used to generate iodine vapor which was introduced into the diffusion plenum to be carried into the bed with the argon stream. In the bed iodine reacted with the silicon to form silicon tetraiodide vapor which upon being elevated to its equilibrium temperature decomposed, depositing :silicon on the part, which silicon reacted With the molybdenum base to form a molybdenum disilicide diffusion complex.

It is uncertain whether the vapor decomposed to deposit silicon on the part, the two subsequently reacting to form the complex; or whether the vapor deposited on the part and .decomposed thereupon `as the ysilicon reacted with the base to yform the complex. nascent silicon was released -in one or the other ways, and it was this nascent silicon which reacted with the part. In any case, a coating identified by X-ray diffraction as molybdenum disilicide was formed.

The thickness of the coating was found to be a function of the time and the temperature of the coating operation, as established through coating operations conducted on similar specimens between 1475 F. and 215 0 F. over times ranging from 5 minutes to l2 hours. The coatings were smooth, uniform, and adherent. Their color varied from metallic-grey to blue-grey .with increasing temperature.

The exhaust gases fromfthe bedfacility, containing argon, iodine, and excess silicon tetraiodidewere carried over into a contact condenser Where the Sil., condensed. The argon and iodine were then passed into a scrubber assembly where a sodium hydroxide-sodium thiosulphate solution was used to wash the gases and remove the iodine.`

Screening oxidation tests were conducted onthe specimens, utilizing both a Burell tube furnace (2700 F.) and an oxyacetylene torch (3000 F.). Scaled `down parts were also tested in a radiant lamp facility at both 2700 F. and 3000 F. The emittance of the coatings was measured at temperatures from 2000 F. to 3000 F utilizing a 36-inch searchlight facility. Specimens ranging in thickness upward from 0.4 mil/side withstood the Burell tube furnace temperature for at least 30 minutes without failure and the oxyacetylene torch'test for l0 minutes or more without failure. 1.0 mil/ side specimens withstood a torch test (2700o F.) for as much as 4 hours without failure and withstood 3000 F. for at least 30 minutes. Reliability was considered as the most significant factor with regard to the thickness of the coat. For purposes of withstanding expected skin temperatures in space vehicles and missiles, a 1.4i0.2 mil thickness/ side was considered to be sufficiently reliable. The emittance of the specimens approximated 0.9 at 2800 F.

Complex shapes such as the ribbed leading edge of an aerodynamic structure were also tested and found to be uniformly coated throughout.

Example II In the earlier described tests of Example I a 0.7 mil/ side coat was obtained at 1800 F. on a Mo 0.5% Ti specimen over a period of 4 hours. Another 0.7 mil/side coat was obtained on a similar specimen at 1500o F. in 10 minutes. On being subjected to oxidation tests, the 1500 F. specimen failed in 15' seconds; The 1800 F. specimen exceeded 30 minutes at 3000" F.

In accord with the duplex process of the invention, a Mo 0.5% Tiy specimen was coated at 1500 F.4 for 10 minutes to obtain a 0.7 mil/ side thickness and was thereafter treated at l800 F. for 30 minutes. On testing,

the duplex specimen exceeded 30 minutes at 3000 F.V The 1800 F. treatment did not require the introduction of halogen gas to the bed facility but apparently relied only on thermal treatment to effect the conversion to the more resistant-form. The 1800 F. treatment, with or without halogen flow, caused no appreciable change in the coating thickness, which is understandable in view of the fact that coating rate decreases as coating thickness increases and at a 0.7 mil/side thickness, the 1500 F. operation had already exceeded the coat that could be gotten at 1800 F. in 30 minutes.

The oxidation resistance characteristics of the duplex coat were substantially the same as those characterizing the single stage coat at 1800 F. in Example I. Thermal treatment of less than 30 minutes in the second stage wili confer these characteristics on the coat.

Example III Tungsten, tantalum, columbium, and zirconium were also siliconized in accord with the procedure used in Example I.

It is possible that The coatings obtained were smooth, uniform, and adherent, and proved to be adequate for the purposes outlined above.

Example IV Certain alloys of the refractory metals were also tested, including Fansteel FS-82 (columbium, 33% tantalum, and 0.7% zirconium); Dupont D36 (columbium, 10% titanium, 5% zirconium); and Waahchang C-l03 (columbium, hafnium). The siliconization coatings all proved to be adequate for the purposes intended for them.

Example V In accord with another feature of the invention, a study was made of the effects of preplating a refractory metal base with nickel, before siliconizing the base.

Columbium was chosen as the base. As with the other refractory bases, the disilicide coating is a time and temperature dependent diffusion process. A columbium disilicide coating thickness of 0.0015 inch/side is normally required to obtain protection at 3000 F. for over 10 minutes. This thickness can be obtained by fluidized bed coating at 1800 F. for approximately 4 hours, 1900 F. for 3 hours, or 2000 F. for 1 hour. The precise coating time depends upon the alloy of columbium or the purity of the columbium being coated. The oxidation resistance is again a function of the coating thickness.

To obtain a more rapid coating rate, nickel was electrolytically plated on columbium metal and the composite was then disilicide coated in a fluidized bed. The coat was formed over a period of 30 minutes at 1800 F. A

v0.006 inch/,side coating thickness was obtained, as compared with a 0.0006 inch/side normal coating thickness obtained at the same temperature and time. This represented a ten-fold increase in the coating rate, with the application of the nickel film. The oxidation resistance of this coating, identified as a columbium disilicide by X-ray diffraction, was equivalent to the oxidation resistance of the slower diffusion rate coating of the same thickness.

Other metals including chromium, iron and cobalt can be used to obtain the same effect. Similarly, all of the refractory metals and especially zirconium, tantalurn, tungsten, and molybdenum experience an increased coating rate as a consequence of this pretreatment step.

Example Vl The oxidation resistance of three columbium alloys, IIS-82, C-103, and D-36, was compared with that of Mo 0.5% Ti at 2700 F. and 3000 F. The columbium alloys were coated at 1900 F., varying the coating time to obtain equal coating thickness of 1.45 to 1.65 mils/ side. The Mo 0.5% Ti specimen was duplex coated to achieve a thickness of 1.45 mils/side. Five l X ll/z coupons of each alloy were placed under radiant heat lamps. Test #l consisted of heating each to 2700" F. at 10 F./

second, holding at this temperature for 30 minutes, and

cooling therefrom at 10 F./seoond. Test #2 consisted of heating at 10 F./second to 3000 F., holdingat this temperature for 10 minutes, and cooling at the same rate. Observations were made during land after each test and the test was repeated until coupon failure was noted. ln the case of both tests, the MoSi2 specimens withstood the oxidation temperatures for considerably longer periods than the columbium alloys.

Molybdenum disilicide products were similarly compared with the disilicide products of commercially pure specimens of other refractory metals. In all instances the molybdenum product proved to be superior, particularly in view of its susceptibility to the duplex process.

While the invention has been described with reference to certain preferred forms thereof, it is to be understood that these are susceptible to many modifications and additions without departing from the scope and spirit of the invention as defined in the following claims.

We claim as our invention:

1. The process of coating a refractory metal article with a silicide diffusion complex thereof, comprising immersing the article in a bed of fluidized particulate material while permeating the bed interstices around the article with a vaporized thermally decomposable compound of silicon, and maintaining the bed in the fluidized condition about the article while adjusting its temperature until the compound decomposes and forms a coating of the complex on the article.

2. The process according to claim l wherein the vaporized compound is a thermally decomposable halide compound of silicon.

3. The process according to claim 2 wherein the silicon is contained in the particulate material and the vaporized compound of the same is produced by introducing a halogen gas into the bed at a temperature which causes the gas to react with the silicon in the particulate material to form the halide thereof.

4. The process according to claim 3 wherein the particulate material is fiuidized by passing a gas upwardly through the bed, and the halogen gas is introduced into the bed with the fluidizing gas.

5. The process according to claim 1 wherein the refractory metal of the article is selected from the group consisting of molybdenum, tungsten, tantalum, columbium, zirconium, chromium, titanium, vanadium, and

hafnium.

6. The Process according to claim 5 wherein the refractory metal is molybdenum.

7. The process according to claim 1 wherein the article is preplated before the coating operation with a metal selected from the group consisting of cobalt, nickel, chromium, an'd iron. v

8. The process according to claim 7 wherein the article is a columbium article and is preplated with nickel before the coating operation is begun.

9. In the coating of a molybdenum article with a silicide diffusion complex thereof, the steps of immersing the article in a bed of fiuidized particulate material While permeating the bed interstices around the article with a vaporized halide compound of silicon, maintaining the bed in the uidized condition about .the article While adjusting its temperature to the range of 1475-1600 F., so as to decompose the compound and form a coating of the silicide diusion complex on the article, and thereafter adjusting the temperature of the bed to a temperature of at least 1800 F. to convert the coating to the'disilicide diffusion complex of molybdenum.

10. The process according to claim 9 wherein the coating operation is discontinued while the coating is converted to the disilicide complex.

References Cited by the Examiner UNITED STATES PATENTS 1,853,370 4/1932 Marshall 117-21 2,665,998 l/l954 Campbell et al. 117--106 2,683,305 7/1954 Goetzel 117-22 2,689,807 9/1954 Kempe et al. 117-107.2 2,978,316 4/ 1961 Weir.

3,053,704 9/ 1962 Munday.

MURRAY KATZ, Primary Examiner.

RICHARD D. NEVIUS, WILLIAM D. MARTIN, R..S.

KENDALL, Assistant Examiners. 

1. THE PROCESS OF COATING A REFRACTORY METAL ARTICLE WITH A SILICIDE DIFFUSION COMPLEX THEREOF, COMPRISING IMMERSING THE ARTICLE IN A BED OF FLUIDIZED PARTICULATE MATERIAL WHILE PERMEATING THE BED INTERSTICES AROUND THE ARTICLE WITH A VAPORIZED THERMALLY DECOMPOSABLE COMPOUND OF SILICON, AND MAINTAINING THE BED IN THE FLUIDIZED CONDITION ABOUT THE ARTICLE WHILE ADJUSTING ITS TEMPERATURE UNTIL THE COMPOUND DECOMPOSES AND FORMS A COATINGS OF THE COMPLEX ON THE ARTICLE.
 7. THE PROCESS ACCORDING TO CLAIM 1 WHREIN THE ARTICLE IS PREPLATED BEFORE THE COATING OPERATION WITH A METAL SELECTED FROM THE GROUP CONSISTING OF COBALT, NICKEL, CHROMIUM, AND IRON. 